The present invention generally relates to pneumatic control apparatus, and more particularly to apparatus for providing an analog pneumatic output control pressure as a function of applied electrical signals.
In the art of heating, ventilating and air conditioning systems (HVAC), there are many systems which utilize pneumatic pressure control lines that extend between components of the system for controlling the operation of the system. The use of such pneumatic lines has existed for decades and systems using the same continue to be installed. As a result of the long use of such pneumatic control lines, there are thousands of systems in existence which may need to be upgraded. However, whether it is for the purpose of upgrading an existing system or interconnecting components in a new system, there is a continuing need for a regulating apparatus which reliably operates to maintain a desired pneumatic pressure in a control line for the purpose of accurately controlling the components in the system. The above cross-referenced and related application of Hurmi et al., Ser. No. 07/714,147 describes the use of pneumatic control lines in an application relating to unit ventilators.
Even though such pneumatic control lines extend to and operatively control valves, dampers and the like, which are associated with a heating and ventilating system, the modulation or regulation of the pneumatic pressure in the pneumatic control lines may be performed by electrically actuated solenoid valves which interconnect the controlled pneumatic output line with a pneumatic supply line of increased pressure and with a low pressure line or ambient atmospheric pressure. Such solenoid valves admit pressure from the supply line to the controlled line to increase the pressure and bleed pressure from the controlled line when the pressure is to be reduced.
One of the problems with such pneumatic control systems is that dust or dirt or other contaminants that may be introduced into the pneumatic control line, which may accumulate and clog small orifices that are necessary in such systems so that the pressure change in the control line will not fluctuate wildly in response to one of the solenoid valves being operated for a short time. Because of the potential for the contaminants clogging the system and thereby severely detrimentally affecting the operation of the system, filters are utilized which catch the dirt particles before they reach the small orifices. While such filters are generally effective, over a period of time they often become loaded and if there is a sufficient amount of dirt present, then the capability of passing air through them is diminished which may also detrimentally affect the operation of the system. This is generally experienced in terms of system response, i.e., when correction of the pneumatic pressure is necessary, it may take longer for the desired control pressure to be obtained in the controlled pneumatic output line. Thus, with many systems, it is necessary to service the control device and clean or replace the filters to regain optimum operation of the system. Given the fact that many systems have many devices with such filters, the maintenance costs can become significant.
Accordingly, it is an object of the present invention to provide an improved output pressure control apparatus of the general type which admits a higher supply pressure to a controlled output line when it is desired to increase the pressure in the output line and which bleeds pressure from the controlled output line when it is desired to reduce the pneumatic output pressure.
Another object of the present invention is to provide such an improved control apparatus which has the superior operating characteristics and also includes a filter means, which by virtue of the superior design of the apparatus, seldom requires cleaning of the filters.
Still another object of the present invention is to provide an improved pressure regulating apparatus of the foregoing type which requires only a single filter means and utilizes bidirectional flow of air through the filter means during normal operation which cleans the filter means.
Another object of the present invention is to provide a pneumatic control apparatus of the foregoing type, which employs a single restrictor, thereby reducing the probability that detrimental clogging of the apparatus will result.
Still another object of the present invention lies in the provision of the choice of normally opened and normally closed solenoid valves which provide maximum safety for such systems, in that a power failure will cause the system output pressure to drop to atmosphere.
Another general object of the present invention is to provide such an improved pneumatic output apparatus which is compact in size, reliable in its operation and has a modular design which facilitates installation and maintenance.
Yet another object of the present invention is to provide such a pneumatic regulating apparatus which includes an easily accessible orifice plate which can be replaced with plates having different orifice sizes to accommodate the design characteristics of the particular system in which it is installed.